It's still anti-government extremists (who are far more likely to come from the far right than the far left) who are the biggest worry, according to a national database of police departments:
In a survey we conducted with the Police Executive Research Forum last year of 382 law enforcement agencies, 74 percent reported anti-government extremism as one of the top three terrorist threats in their jurisdiction; 39 percent listed extremism connected with Al Qaeda or like-minded terrorist organizations. And only 3 percent identified the threat from Muslim extremists as severe, compared with 7 percent for anti-government and other forms of extremism.
Last year, for example, a man who identified with the sovereign citizen movement — which claims not to recognize the authority of federal or local government — attacked a courthouse in Forsyth County, Ga., firing an assault rifle at police officers and trying to cover his approach with tear gas and smoke grenades. The suspect was killed by the police, who returned fire. In Nevada, anti-government militants reportedly walked up to andshot two police officers at a restaurant, then placed a “Don’t tread on me” flag on their bodies. An anti-government extremist in Pennsylvania was arrested on suspicion of shooting two state troopers, killing one of them, before leading authorities on a 48-day manhunt. A right-wing militant in Texas declared a “revolution” and was arrested on suspicion of attempting to rob an armored car in order to buy weapons and explosives and attack law enforcement. These individuals on the fringes of right-wing politics increasingly worry law enforcement officials.
(T)errorism of all forms has accounted for a tiny proportion of violence in America. There have been more than 215,000 murders in the United States since 9/11. For every person killed by Muslim extremists, there have been 4,300 homicides from other threats.
Update, June 19: Dylann Roof is the latest proof of how true this all is.